1. Field of the Invention
The present invention relates to a structure of a motor and more particularly to a structure of a synchronous motor and a stepping motor of small size.
2. Description of the Prior Art
Small-sized synchronous motors and stepping motors are being used as drive sources of various electronic apparatuses such as the drive source of a floppy disk drive as an item of peripheral equipment of a personal computer. Of such motors, there is an inner rotor type, in which a cylindrical rotor constructed of magnetic poles of a permanent magnet is disposed for rotation and stator magnetic poles, which generate a revolving magnetic field to provide the rotor with a driving force, are disposed around the rotor in confronting relationship. The stator magnetic poles are fixed onto a substrate constructed of an insulated metal plate and the rotor is borne by two bearings respectively fixed to the substrate and the stator magnetic poles. Further, there are disposed such parts as electronic circuits for driving the motor and a position detecting coil for detecting the position of the rotor fixed onto the substrate.
There is also a motor of an outer rotor type, in which a plurality of magnetic poles is radially projected from a yoke member in a circular ring form, stator coils are wound around the magnetic poles, and the thus formed stator magnetic poles are fixed onto a substrate, and on the other hand, a rotating plate of a shallow dish-form is disposed around the stator magnetic poles for rotation around the same with permanent magnets disposed on the inside of the circumferential portion of the rotating plate arranged in confronting relationship with front ends of the magnetic poles constituting the stator.
In the assembly work of either of the motor type, such an intermediate product is used for the stator magnetic poles that is formed by laminating a plurality of sheet members punched from a soft iron plate and having the laminated sheet members put together by spot welding, caulking, or by passing a bar through the laminations and then caulking both ends of the through bar. Such an intermediate product is called a lamination stack.
For the small-sized synchronized motor or stepping motor in which the rotor rotates with a minute gap between it and the stator magnetic poles, the machining accuracy of the component parts has great importance for good performance. Especially, occurrence of noise of the motor is greatly dependent on accuracy of the lamination stack.
In order to manufacture them at low cost or improve the manufacturing efficiency, conventional lamination stacks have been manufactured by caulking "V" or "O" shaped caulked portions provided in each lamination (sheet member), in a stator iron core of a predetermined thickness within a press metallic mold. However, it is difficult to maintain the inner diameter of the lamination and the caulked portion coaxial, which is important for the accuracy of the motor, and it has been unavoidable that "deviation in lamination" occurs in the inner diameter of the stator iron core joined by caulking. Also, due to bulging of the base portion of the "V" caulking, a small gap is produced between laminations to deteriorate the tight contact between them and, hence, it has been impossible to increase the magnetic flux density and, accordingly, to increase the torque.
It is also possible to obtain a lamination stack by laminating sheet members, each of which has been punched out with high precision and bonding them on top of each other. However, since there is a problem with the bonding strength of the bonding agent, it has been impossible to obtain a durable lamination stack providing a good performance of the motor. In the case of an HB type stepping motor, it is practiced to place two robust covers made of cast iron at the top and the bottom sides of the lamination stack and allow the covers to strongly press the lamination stack from the top and the bottom sides by such means as fastening of screws so that the lamination stack will not cause deviation in lamination with time or the lamination stack will not cause displacement or deviation in lamination only by being subjected to a certain amount of vibration or impulse. However, since the covers are made of a soft magnetic material and have a large volume, and, accordingly, have low magnetic reluctance, there is such a disadvantage that the covers allow a lot of leakage fluxes from the stator magnetic poles to pass through the cover.
The present invention was made to overcome the above mentioned difficulties included in the related art. Accordingly, it is an object of the invention to provide a lamination stack keeping the laminated sheets in good contact with each other, maintaining the accuracy of the lamination stack over a long time of service, and being durable.